Conventionally, optical semiconductor devices such as semiconductor lasers are used in the field of communications. With the proliferation of the Internet and other communications networks, the amount of communication traffic has been increasing and, to address the increasing amount of traffic, work aimed at increasing communication speeds and communication capacities in optical communications or optical transmissions using such optical semiconductor devices has been proceeding.
For example, there has developed a need for an optical fiber transmission system that achieves a communication speed of 40 Gb/s or higher or for a system capable of transmitting data amounting to 100 gigabits in traffic by bundling together four waves, each 25 gigabits of data, using a LAN-WDM.
It is believed that semiconductor lasers can be used to generate optical signals for such communications by direct modulation. One example of such a semiconductor laser is a distributed feedback laser (DFB laser) which incorporates a diffraction grating in the active region.
To perform direct modulation at high frequency using a DFB laser, a high relaxation oscillation frequency is needed. In the DFB laser, the relaxation oscillation frequency can be increased by reducing the volume of the active region of the semiconductor laser. For example, there is proposed a DFB laser capable of modulating at 40 Gb/s at room temperature by reducing the length of the laser resonator to 100 μm.
On the other hand, in order to increase the efficiency and the relaxation oscillation frequency of the DFB laser and to cause it to oscillate in a single longitudinal mode, there is proposed a distributed reflection laser (DR laser) equipped with distributed reflection mirrors and having a phase shift portion and a diffraction grating for causing light produced by the active region to be reflected back into the active region.
FIG. 1 is diagram illustrating an optical semiconductor device as a DR laser.
The optical semiconductor device 110 is a DR laser. The optical semiconductor device 110 includes a first distributed reflection mirror region 110a, an active region 110b, and a second distributed reflection mirror region 110c. 
The active region 110b includes a substrate 111, a first lower cladding layer 112 formed on the substrate 111, a first diffraction grating layer 113 formed on the first lower cladding layer 112, and a grating phase shift portion 114 (hereinafter, “a phase shift” means “a grating phase shift” in the present specification and drawings) formed within the first diffraction grating layer 113 and providing a phase shift of π radians. A second lower cladding layer 115 is formed on the first diffraction grating layer 113, an active layer 116 is formed on the second lower cladding layer 115, and an upper cladding layer 117 is formed on the active layer 116. A first electrode layer 120 is formed on the upper cladding layer 117, and a second electrode layer 121 is formed on the underside of the substrate 111. The first and second electrode layers 120 and 121 act to inject current into the active layer 116. The substrate 111, the first lower cladding layer 112, the second lower cladding layer 115, and the upper cladding layer 117 each extend into the first and second distributed reflection mirror regions 110a and 110c. 
The first and second distributed reflection mirror regions 110a and 110c each include a second diffraction grating layer 122 formed on the first lower cladding layer 112, the second lower cladding layer 115 formed on the second diffraction grating layer 122, and an optical guide layer 123 formed on the second lower cladding layer 115. The second diffraction grating layer 122 is formed integrally with the first diffraction grating layer 113, and has a diffraction grating having the same grating period as that of the first diffraction grating layer 113. The optical guide layer 123 is optically coupled to the active layer 116.
A first antireflection layer 128 is applied to cover an open end of the first distributed reflection mirror region 110a, and a second antireflection layer 129 is applied to cover an open end of the second distributed reflection mirror region 110c. 
The active layer 116 in the active region 110b produces light when current is injected into it. The first and second distributed reflection mirror regions 110a and 110c on both sides of the active region 110b cause the light produced by the active region 110b to be reflected back into the active region 110b. The active region 110b produces laser light having a wavelength with a period twice that of the diffraction grating formed in the first diffraction grating layer 113. The laser light produced by the active region 110b propagates through the optical, guide layers 123 on both sides and is output from both end faces of the optical semiconductor device 110.
International Publication Pamphlet No. WO2010/100738
Japanese Laid-open Patent Publication No. 2000-58970
K. Nakahara et al., “High Extinction Ratio Operation at 40-Gb/s Direct Modulation in 1.3-μm InGaAlAs-MQW RWG DFB Lasers,” OFC/NFOEC 2006, OWC5